Chimpanzees Form Trusting Friendships, Not Unlike Humans

Chimpanzees trust their friends more than non-friends, a study has shown. Malin Palm / REUTERS

If you thought that humans were the only animals that could form friendships and trust one another, think again.

A study published Thursday in the journal Current Biology found that chimpanzees trust their friends significantly more than those to whom they aren't particularly close.

In the paper, the researchers examined the interactions of 15 chimpanzees in the Sweetwaters Chimpanzee Sanctuary in Kenya over a five-month period, recording the amount of time the primates spent grooming one another and eating together, to quantify how close various pairs were.

They then matched up chimps with their friends and non-friends, in a two-player activity called "the trust game." In it, each participant has a choice between two options, explains study co-author Jan Engelmann, of the Max Planck Institute for Evolutionary Anthropology in Germany: The chimp can pull the "no-trust rope," which opens a compartment that gives the puller access to a small but not-enormously-desirable amount of food (two banana pieces). Or it can pull the "trust rope," which gives the other chimp access to a tastier and larger food cache (three banana pieces and three apple slices), with the important caveat that the second chimp has the option of giving some of it back. So if the first chimp trusts the second, it makes sense to pull the ol' trust rope—if the second chimp is friendly enough, it may result in getting a good snack back versus a mediocre one.

Engelmann and colleague Esther Herrmann did a statistical analysis that showed a strong link between whether the chimps were friends and whether or not they pulled the trust rope. In other words, friends were much more likely to trust each other. If the chimps didn't have something like trust-based friendships, they would be likely to behave similarly toward friends and non-friends. But the animals consistently acted in a way that defied this expectation.

"This is an unexpected finding, as it suggests that chimpanzees do not regulate their behavior toward partners exclusively as a function of received rewards," Engelmann says. "Instead, they seem to form deep emotional bonds with their friends, which are, at least to some extent, independent of short-term payoffs."

The study shows that "long before humans started to develop culturally mediated forms of friendships, our ancestors likely engaged in many of the patterns…of modern human bonds," he adds.